1. Field
The present disclosure relates generally to geodesy and precise positioning of a handheld geodesic device.
2. Related Art
Navigation receivers that use global navigation satellite systems, such as GPS or GLONASS (hereinafter collectively referred to as “GNSS”), enable a highly accurate determination of the position of the receiver. The satellite signals may include carrier harmonic signals that are modulated by pseudo-random binary codes and that, on the receiver side, may be used to measure the delay relative to a local reference clock. These delay measurements may be used to determine the pseudo-ranges between the receiver and the satellites. The pseudo-ranges are not true geometric ranges because the receiver's local clock may be different from the satellite onboard clocks. If the number of satellites in sight is greater than or equal to four, then the measured pseudo-ranges can be processed to determine the user's single point location as represented by a vector X=(x, y, z)T, as well as to compensate for the receiver clock offset.
GNSS finds particular application in the field of surveying, which requires highly accurate measurements. The need to improve positioning accuracies has eventually led to the development of differential navigation/positioning. In this mode, the user position is determined relative to an antenna connected to a base receiver or a network of base receivers with the assumption that the positional coordinates of the base receiver(s) are known with high accuracy. The base receiver or receiver network transmits its measurements (or corrections to the full measurements) to a mobile navigation receiver (or rover). The rover receiver uses these corrections to refine its measurements in the course of data processing. The rationale for this approach is that since the pseudo-range measurement errors on the base and rover sides are strongly correlated, using differential measurements will substantially improve positioning accuracy.
Typically, the base is static and located at a known position. However, in relative navigation mode, both the base and rover are moving. In this mode, the user is interested in determining the vector between the base and the rover. In other words, the user is interested in determining the continuously changing rover position relative to the continuously changing position of the base. For example, when one aircraft or space vehicle is approaching another for in-flight refueling or docking, a highly accurate determination of relative position is important, while the absolute position of each vehicle is generally not critical.
The position of the rover changes continuously in time, and thus should be referenced to a time scale. The determination of the position of a mobile rover with respect to a base receiver in real-time may be performed using an RTK algorithm, which may be stored in memory on the rover. As the name “real-time kinematic” implies, the rover receiver is capable of calculating/outputting its precise position as the raw data measurements and differential corrections become available at the rover. When implementing an RTK algorithm, a data communication link (e.g., a radio communication link, a GSM binary data communication link, etc.) may be used to transmit the necessary information from the base to the rover.
Further improvement of the accuracy in differential navigation/positioning applications can be achieved by using both the carrier phase and pseudo-range measurements from the satellites to which the receivers are locked. For example, by measuring the carrier phase of the signal received from a satellite in the base receiver and comparing it with the carrier phase of the same satellite measured in the rover receiver, one can obtain measurement accuracy to within a small fraction of the carrier's wavelength.
One well-known type of measurement error that can reduce the accuracy of differential navigation/positioning is multipath error. Multipath errors are caused by the reflection of the GNSS satellite signals by surfaces located near the receiving antenna. As a result of these reflections, the antenna receives both the direct signal traveling the shortest path from the satellite to the receiver as well as the reflected signals following indirect paths. The combination of two (or more) signals at the antenna leads to the distortion of raw measurements. Multipath errors may affect both pseudo-range and carrier phase measurements.